One of 4 PC's can't access web but is connecting to network

This is the problem:

I have four PC's on a network using a hub which then connects to my modem. My main PC connects directly into a second port on my modem. All my PC's have had no problem connecting to each other or to the net up until two days ago when I found that my wife's PC in the study will no longer connect to the net, although it is still able to access the network and shows no problem with "Local Connection".

All our PC's run XP and Vista except for hers. She stubbornly refuses to change from Windows 2000 as it is her preferred O/S and for what she does there is no real reason for her to change/upgrade to XP. So - based on that can anyone recommend a way to get her back on the net or what the problem may be?

Cheers.